1. Field of the Invention
The present invention relates generally to automatic code symbol reading systems, and more particularly, to laser bar code reading systems which permit fully automated operation while providing a high degree of simplicity and flexibility.
2. Brief Description of Background Art
Hitherto, a number of techniques have been proposed for reading bar code symbols using hand-held devices. Despite variety amongst prior art bar code symbol reading devices, the various techniques incorporated into prior art devices can be classified into two principally distinct classes, namely, manually operated or triggered bar code symbol reading, and automatic bar code symbol reading.
Representative of prior art manually operated bar code symbol reading devices are U.S. Pat. No. 4,387,297 to Swartz, et al., U.S. Pat. No. 4,575,625 to Knowles, and U.S. Pat. No. 4,845,349 to Cherry. While such prior art devices are capable of bar code symbol reading, they nevertheless suffer from several significant shortcomings and drawbacks. In particular, the user is required to manually pull a trigger or push a button each time symbol reading (i.e., scanning and decoding) is to be cyclically initiated and terminated. This requirement is most fatiguing on the user when large numbers of bar code symbols are to be read. Also, in certain symbol reading applications, such as warehouse inventory, pulling the trigger to initiate scanning of bar code symbols may be extremely difficult for the user due to the physical location of the objects bearing the bar code symbols.
An alternative to manually operated bar code symbol reading devices is automatic bar code symbol readers, which incorporate techniques for automatically initiating and terminating scanning and decoding operations. Representative of prior art automatic bar code symbol devices are U.S. Pat. No. 4,639,606 to Boles, et al. and U.S. Pat. No. 4,933,538 to Heiman, et al. While capable of automatically initiating scanning of bar code symbols, such prior art devices and incorporated techniques nevertheless also suffer from significant shortcomings and drawbacks.
In particular, U.S. Pat. No. 4,639,606 to Boles, et al. discloses laser emission control circuitry for use in implementing a hand-held triggerless bar code scanner. The laser is operated in a pulsed “find paper” mode until a reflected signal is obtained, indicating the presence of an object (e.g., paper) in the search field. Thereupon, the circuitry is changed to a “search mode” in which the power of the laser is increased to above the safety limits for a period of time, and the return signal is monitored for signal transitions corresponding to the black bars of the code. On detection of the first black bar, the circuitry is changed to an “in-code” (i.e., decode) mode as long as successive symbols are received within a given period of time. If the decode mode terminates within a predetermined time interval (e.g., one second after the beginning of the search mode), then the search mode is re-entered, otherwise the decode mode will change to find paper mode.
While the triggerless bar code symbol reader proposed in U.S. Pat. No. 4,639,606 possesses three modes of operation, this prior art bar code symbol reader nevertheless suffers from several significant shortcomings and drawbacks. In particular, this prior art bar code symbol reader requires continuous use of a pulsed laser beam to determine the presence of an object within the scan field, which, in hand-held portable battery power devices, undesirably drains limited power reserves, especially in extended time duration bar code reading applications. Also, this prior art device, not knowing whether a bar code symbol is actually present in the scan field, requires commencement of decode processing upon detection of the first black bar. Undesirably, this typically necessitates initializing a programmable device, such as a microprocessor, for decoding scan data that may likely contain no bar code symbol at all. Consequently, this characteristic of such prior art bar code symbol reading devices results in decreased responsiveness and versatility.
U.S. Pat. No. 4,933,538 discloses a bar code symbol reading system which, in the “object sensor mode”, is triggerless and constantly emits a laser beam at a narrow angle and low power. When an indicia pattern indicative of a bar code symbol has been detected, the laser beam is widened and its power increased, for reading the entire symbol. While this prior art bar code reading system permits detection of bar code symbols within the scan field in order that the power of the laser beam may be automatically increased to a higher level for collecting scan data for use in decoding operations, this system also suffers from several significant shortcomings and drawbacks. In particular, it requires continuous use of laser emission to determine the presence of both objects and bar code symbols within the scan field, which necessarily results in drain of limited power reserves in portable battery power applications. In addition, the extensive use of a laser beam to perform object and bar code symbol detection functions implicates necessity for laser emission control measures.
Prior art automatic bar code symbol reading devices, such as the devices described above, suffer from other shortcomings and drawbacks. At the outset, consider manually operated scanners. These utilize a triggering mechanism that must be activated every time the user desires to read a bar code. But, if this process is somehow automated to eliminate the need for trigger activation, other potential problems are created. For example, prior art automatic bar code symbol reading devices lack the necessary intelligence capabilities to prevent undesired multiple reading of a bar code symbol, particularly when the scanning beam is permitted to dwell on a bar code symbol for an extended period of time.
Further, prior art automatic bar code symbol reading devices lack system control capabilities which permit diverse modes of operation and automatic reading of a plurality of consecutively different bar code symbols, while preventing misreads and inadvertent multiple reads of the same bar code symbol.
While prior art manually-triggered and prior art CCD (charge-coupled display)-type scanners have played an important role in the development of the bar code symbol industry, these devices, suffer from a number of shortcomings and drawbacks. For example, hand-held manually-actuated laser scanners, although portable and lightweight, are not always convenient to use particularly in applications where the user must read bar coded objects over an extended period of time. In many applications, where bar coded objects to be identified reside at arms length from the user's reach, hand-held CCD scanners are difficult to operate owing to their limited depth of field.
Unlike manually automated hand-held bar code scanners, fully automatic hand-held laser scanners do not cause fatigue due to their automatic operation. Also, owing to their extended depth of field, automatic hand-held laser scanners provide increased flexibility by allowing the user to read bar coded objects residing at distances of six or more inches away from the scanner. However, even though automatic hand-held laser bar code scanners offer superior performance in most scanning applications, it has been found that in intensely illuminated scanning environments, the user's ability to perceive the visible laser scanning beam is significantly diminished in the scan field of the device. Consequently, in such scanning environments, it is difficult to visually align (i.e. register) the laser scanning beam with the bar code symbol to be scanned, thus hindering the automatic bar code symbol reading process. While the use of a higher power visible laser beam might render the beam more easily perceptible, this approach is undesirable for laser safety and power consumption reasons.
Thus, there is a great need in the code symbol reading art for a fully automatic hand-holdable code symbol reading device which overcomes the above shortcomings and drawbacks of prior art devices and techniques.